The present invention relates to metal-air batteries of the type that are supplied with reactive gas by an active air moving device, and more particularly relates to an air mover mechanism that utilizes a diaphragm or bellows to move air in and out of one or more air openings or to move air from an inlet to an outlet.
Generally described, a metal-air cell, such as a zinc-air cell, uses one or more air permeable cathodes separated from a metallic zinc anode by an aqueous electrolyte. During operation of the cell, oxygen from the ambient air is converted at the one or more cathodes to produce hydroxide ions. The metallic zinc anode is then oxidized by the hydroxide ions. Water and electrons are released in this electrochemical reaction to provide electrical power.
Initially, metal-air cells found limited commercial use in devices, such as hearing aids, which required a low level of power. In these cells, the air openings which admitted air to the air cathode were so small that the cells could operate for some time without flooding or drying out as a result of the typical difference between the outside relative humidity and the water vapor pressure within the cell. However, the power output of such cells was too low to operate devices such as camcorders, cellular phones, or laptop computers. Furthermore, enlarging the air openings of a typical xe2x80x9cbutton cellxe2x80x9d was not practical because it would lead to premature failure as a result of flooding or drying out.
In order to increase the power output of metal-air cells so that they could be used to operate devices such as camcorders, cellular phones, or laptop computers, air managers were developed with a view to providing a flow of reactive air to the air cathodes of one or more metal-air cells while isolating the cells from environmental air and humidity when no output is required. As compared to conventional electrochemical power sources, metal-air cells containing air managers provide relatively high power output and long lifetime with relatively low weight. These advantages are due in part to the fact that metal-air cells utilize oxygen from the ambient air as the reactant in the electrochemical process as opposed to a heavier material such as a metal or a metallic composition. Examples of air managers are shown in U.S. Pat. Nos. 4,913,983, 5,356,729, and 5,691,074.
A disadvantage of most air managers, however, is that they utilize an air moving device, typically a fan or an air pump, that occupies space that could otherwise be used for battery chemistry to prolong the life of the battery. This loss of space presents a particular challenge in attempts to provide a practical metal-air cell in small enclosures such as the xe2x80x9cAAxe2x80x9d cylindrical size now used as a standard in many electronic devices.
In addition to being bulky, air moving devices used in metal-air batteries also consume energy stored in the metal-air cells that might otherwise be delivered as power output to a load. Complicated electronics for controlling an air manager can increase this use of stored energy. Also, as most air moving devices used in metal-air cells distribute air to a cathode plenum at low pressure, a flow path must be defined passing over all regions of the cathode surface to evenly distribute air to the entire cathode surface. Thus, the function of bringing in make up air and the function of mixing and distributing air within the battery have been separate. A further disadvantage of fans used as air moving devices in metal-air cells is that they may create noise at a level disruptive to users of devices such as cellular telephones.
As a result, while a key advantage of metal-air cells is their high energy density resulting from the low weight of the air electrode, this advantage is compromised by the space and power required for an effective air manager, and the noise it may produce.
Therefore, there has been a need in the art for an air manager incorporating an air moving device that occupies less of the volume available for battery chemistry, is usable with advanced systems for isolating the air electrodes when power is not being drawn from the metal-air cell, is quiet, does not require a complex baffle system in the cathode air plenum to distribute the air, needs relatively simple controls, and consumes power at a relatively low rate.
The present invention seeks to provide an improved air moving device for metal-air cells that occupies a minimal amount of the volume available for battery chemistry, is usable with advanced systems for isolating the air electrodes when power is not being drawn from the metal air cell, and is capable of developing high velocity air movement at a relatively low rate of power consumption.
In accordance with one aspect of the invention, this object is accomplished by providing an air-moving device for supplying ambient air to the air electrodes of a metal-air cell by moving the air alternately in through and out of a passageway extending from the air electrodes to an outside air environment.
In a preferred embodiment, the air moving device is a diaphragm or bellows reciprocated by a linear actuator, such as an electromagnetic oscillator, or a shape memory alloy wire. There may be one or more passageways preferably including an isolating passageway such as a thin elongate tube shaped to impede air flow to the air electrode when the air moving device is not operative, even while the tube remains unsealed. In this embodiment, the function of bringing in make up air is combined with the function of circulating and mixing the air for the metal-air cell or cells by giving the make up air stream sufficient pressure and velocity to provide mixing and circulation. Make up air entry points can be located to make use of the inertial force of the air stream along with diffusion and thermal forces to perform the circulation and mixing function. Furthermore, the air movers of this embodiment can have simplified controls and power requirements that use up less than 5% of the energy stored in the battery. In particular, when the actuator is a shape memory wire or electromagnetic oscillator, controls needed to supply a fixed voltage and the attendant voltage conversion and regulation needed to run fans and blowers may be eliminated.
According to another of its aspects, the present invention provides a reciprocating partition for moving air and one or more ventilation passageways extending through the partition and operable for providing outside air to a metal-air cell as the partition reciprocates. The ventilation passageway may be a tube attached at one end to an opening in the partition, the tube reciprocating with the partition. In a preferred form, the partition is a rolling diaphragm.
According to another of its aspects, the present invention provides a ventilation system for a metal-air power supply, having one or more cells each including an air electrode, and at least one air passageway passing between facing surfaces. Each surface defines an opening therethrough and the openings through the surfaces are spaced apart from one another. The passageway is capable of passing sufficient air to operate the cell when associated with an operating air moving device, and the passageway is further operative, while unsealed and not under the influence of an operating air moving device, to restrict air flow through the passageway to protect the cells. Preferably, one of the facing surfaces is a movable diaphragm, and the air moving device includes the diaphragm and an actuator for reciprocating the diaphragm. When the air moving device is not reciprocating the diaphragm, the diaphragm is positioned in a rest position closely adjacent to the other facing surface. In the rest position, the surfaces preferably are, for practical purposes, touching in the region between their respective openings, the centers of which preferably are spaced apart along the surfaces by at least about 1.5 times the diameter of the openings. The surfaces need not, however, be touching, as long as the openings are separated far enough apart, depending on the size of the gap between the surfaces, to retard passage of air between the openings in a manner similar to that provided by the isolation passageways described herein.
According to another of its aspects, the present invention provides in a metal-air battery including a cylindrical housing containing a pair of metal-air cells and a pair of facing air cathodes separated by a generally rectangular cathode air plenum, an air manager comprising an air pathway defined within the housing by the cathode air plenum connected at one end of the housing to a return plenum defined between a chordal wall and a cylindrical wall of the housing. An air moving device is operable to move a flow of air axially through the cathode air plenum and axially in the opposite direction through the return plenum. The housing may further include an air inlet, preferably an isolating tube, providing outside air to the air moving device and an air outlet, also preferably an isolating tube, directing at least a portion of air moving through the return plenum to the exterior of the battery. The air moving device may be a diaphragm reciprocating within a peripheral guide aligned with the cathode plenum.
According to another of its aspects, the present invention provides in a metal-air battery including a housing containing one or more metal-air cells each having an air electrode, an air manager comprising an air pathway defined within the housing and extending adjacent to an air electrode of a metal-air cell; an inlet and an outlet each extending between the air pathway and an environment outside the housing; a micro-oscillator mounted in the air pathway; and a diaphragm air pump connected to the micro-oscillator, the micro-oscillator vibrating the diaphragm to move air along the air pathway between the inlet and the outlet. The micro-oscillator may oscillate the diaphragm at a frequency of 20,000 hertz or greater so that the frequency will be above those normally audible to the human ear. The battery may also define a recirculation path positioned to cause a portion of the air flowing in the air pathway to bypass the outlet.
In a preferred form of this embodiment, the battery includes a pair of metal-air cells and a pair of facing air cathodes separated by a generally rectangular air plenum that includes a central axis of a cylindrical housing. The air pathway is a U-shaped space defined by a cathode current collector extending to divide the air plenum for a portion of the distance across the air plenum.
According to another of its aspects, the present invention provides an air moving device for a metal-air battery including one or more metal-air cells, comprising a flexible membrane, a leaf spring extending across at least a portion of the membrane and attached thereto, a shape memory wire attached at its ends to the leaf spring so as to lie loosely along the leaf spring when the wire is in a relaxed condition; and a circuit selectively connecting the ends of the wire to the cell to direct a current through the wire, causing the wire to shrink, thereby bending the leaf spring, and deforming the membrane to move air within the battery.
The air moving devices and air pathways of the various embodiments of the present invention provide improved air managers for metal-air batteries including one or more metal-air cells. As may be understood from the foregoing, most aspects of the present invention are applicable to individual metal-air cells or to batteries of cells, and to both prismatic and cylindrical cells and batteries.
Other objects, features and advantages of the present invention will become apparent upon reviewing the following detailed description of preferred embodiments of the invention, when taken in conjunction with the drawings and the appended claims.